Valve proof take a look at credit for a course of trip

A course of trip happens when the security instrumented system (SIS) locations the process in its secure state by commanding an automated valve (Figure 1) or different gear to its trip state in response to an irregular course of condition. In some cases, a spurious journey happens because of a valve malfunction. In that case, the valve meeting is examined under actual operating conditions, which offers an opportunity to capture priceless valve diagnostic information.
However, such diagnostics information can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this article, we’ll focus on how DVCs might help determine the proof check credit for an automated valve after a course of trip.
Process journey

A process trip occurs when the SIS detects an irregular process situation via sensors similar to temperature and strain, executes the logic and locations the method in its secure state by tripping the ultimate components corresponding to closing an automated valve, stopping a pump, etc. The SIS could communicate with the Basic Process Control System (BPCS) to close a valve downstream of the automated valve as a secondary action.
When a process journey occurs, the principle objective is normally to restart the unit or tools that has been shut down as soon as attainable. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing damaged devices. Taking the opportunity to proof check an automated valve won’t be a high priority or even an exercise under consideration as a result of tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is provided with a DVC, the proof take a look at of the valve may be thought of carried out with diagnostic knowledge captured by the DVC. Also, a comparison of the diagnostic knowledge with the valve’s baseline might help discover valve deficiencies and degradations that won’t show up in a proof test.
Figure 2. Proof Test vs Process Trip. Activities carried out (such as repairing the device) and information captured by a Fisher DVC6200 SIS digital valve controller during a process trip meet many of the 12 necessities of a proof take a look at.
Process journeys versus proof tests

How can proof test credits be claimed for an automated valve that has failed and brought on a process trip? A proof take a look at is a periodic take a look at carried out on SIS sensors, logic solvers and ultimate parts — corresponding to automated valves — to find hidden failures. These failures, if not corrected or removed, may impair the flexibility of the SIS to take the process to its protected state when an irregular course of situation is detected.
A proof take a look at should be carried out as per the proof take a look at interval specified in the Safety Requirement Specifications. This interval is normally decided via a median likelihood of failure on demand calculation (PFDavg). Some customers may choose to proof test primarily based on predetermined intervals, similar to proof testing sensors each 24 months and ultimate components every forty eight months as an alternative of counting on the PFDavg calculations.
Automated valve proof checks could be done offline or online. Offline proof exams are usually scheduled during a turnaround, when the method is completely shut down.
Proof testing an automatic valve on-line often requires a unit or equipment to be shut down, or the valve to be bypassed to stop a spurious trip, making it inconvenient and costly.
But a proof check may additionally be completed throughout a process journey.
According to IEC61511-2 Section A.sixteen.3.1.three, “…shutdowns due to precise demand on the SIS during operation may be given credit as proof checks (fully or partial) underneath given conditions…the subsequent planned proof take a look at may be skipped.”

These conditions are

The shutdown paperwork equal data as registered during corresponding proof take a look at.
The shutdown covers all components of the SIS, and if not, the system or SIS subsystem not activated must be tested individually.
The shutdown occurs within a predetermined most time window earlier than the next deliberate proof test which might then be canceled

When a process journey occurs, about 60% of a Safety Instrumented Function (SIF) proof take a look at can be thought of performed. A pattern listing of actions performed throughout a proof check, along with those which may be carried out during a course of trip, is proven in Figure 2. Even with out an automated valve leak check, information captured by the DVC alone can potentially account for a great quantity of proof take a look at coverage for an automated valve.
The precise coverage is dependent upon the design of the automated valve, its DVC and its software. The coverage is estimated primarily based on the valve degradations, the likelihood of their incidence and the proportion of those degradations that may be detected by a DVC. Fortunately, information acquired by a DVC throughout a course of journey can usually be enough to fulfill a major a part of the proof test necessities.
If the process journey takes place inside a predetermined maximum time window, the tip person may select to leverage the process trip as a proof check by finishing steps one by way of 5 in Figure 2, which are usually not accomplished in a course of journey. The next scheduled proof check can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window ought to be the final half of the current proof test interval.
Figure three. Data throughout a process trip is captured by the Fisher DVC6200 SIS digital valve controller and made out there for analysis.
pressure gauge น้ำ of valve failure

According to ISA Technical Report 96.05.01, the most probably root causes of automated valve degradations — which might result in a course of journey — include:
Partial or intermittent sticking of hydraulic or pneumatic system components such as solenoids, pilots, velocity controller, etc., because of moisture, debris or alignment points. This causes a lack of functional margin and makes the valve gradual to open or close.
Binding, galling or different degradation of valve seats or related flow control trim that restricts or resists valve motion.
Actuator seal degradation caused by compression, wear or looseness that reduces the stress obtainable to actuate the valve.
Minor damage to the valve obturator plug, disk or ball attributable to system circumstances, leakage or particles, together with build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic management system elements corresponding to solenoids, pilots, pace controller, and so on., due to moisture, particles or alignment. The valve will fail to open or close.
All of these situations can be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For instance, a valve that’s caught open will exhibit itself as a rise in breakout pressure. Loss of seat load in comparison with when the valve assembly was new indicates potential valve leak. The Fisher DVC6200SIS additionally continuously displays for inner faults as well as its inputs such as provide pressure. When the DVC6200SIS sees the availability strain is too low, an alert is generated (Figure 4) in order that the top user can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected nearly zero supply pressure, so it alerted the management system.
Other course of trip benefits

Diagnostic data captured during a process trip might reveal valve degradations that may not be detected throughout a proof take a look at. For instance, diagnostic data captured during a process trip may point out a problem with the valve closing fully against the total pressure of the method, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a process trip is extra accurate underneath actual working circumstances. This leads to a extra accurate SIF response time (the time from detection of an abnormal situation to final factor reaching its journey state), which is in comparability with the Process Safety Time to ensure the SIF continues to be meeting its design necessities.
Although automated valves are very dependable, failures recognized in a course of journey can present valuable knowledge to stop future failures. This data can assist with turnaround planning by making sure the wanted elements can be found earlier than turnaround even begins to potentially shorten the turnaround schedule.

A process journey can provide the coverage required to delay a valve’s next scheduled proof check, thus saving money and time. Data captured by a DVC may be analyzed to leverage the method journey as a proof check. Even if the end person chooses not to take proof take a look at credits for a process journey, the valve diagnostic information supplied by the DVC can help plant personnel make proactive valve maintenance choices..


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